Abstract: We have combined structural and magnetic characterisations to investigate the effect of carbon incorporation in epitaxial Mn5Ge3Cx films grown on Ge(111) by Molecular Beam Epitaxy (MBE). It is shown that up to a carbon content of saturation of ~0.6, most of carbon can be incorporated into the interstitial sites of the Mn5Ge3 lattice. Such a process results in a linear increase in the Curie temperature (TC) of the alloy, which can reach a value as high as ∼430 K. Above this carbon content, TC is found to decrease. Structural characterisations reveal that Mn5Ge3Cx films are in perfect epitaxy when x ∼0.6 whereas cluster formation in the grown layers is detected above that threshold. The clusters can be attributed to manganese carbide (MnC) compounds, which are formed when the carbon content exceeds the saturation value of 0.6 by consuming previously deposited carbon. In addition, we also show that after post-thermal annealing, the carbon-doped Mn5Ge3Cx alloys remain magnetically and structurally stable up to a temperature as high as 1123 K. The obtained results are very promising for integrating Mn5Ge3Cx into ferromagnetic/semiconductor heterostructures, the ultimate goal being the realisation of spintronics devices.